Method of making retroreflective sheeting and articles

ABSTRACT

The invention relates to a method of making retroreflective sheeting and other articles prepared from casting a moldable synthetic resin onto a tool having a microstructured surface.

RELATED APPLICATIONS

[0001] This application claims priority to U.S. Patent ApplicationSerial No. 60/452605 filed Mar. 6, 2003.

FIELD OF THE INVENTION

[0002] The invention relates to a method of making retroreflectivesheeting prepared from casting a moldable synthetic resin onto a toolhaving a microstructured surface.

BACKGROUND OF THE INVENTION

[0003] Retroreflective materials are characterized by the ability toredirect light incident on the material back toward the originatinglight source. This property has led to the widespread use ofretroreflective sheeting for a variety of traffic and personal safetyuses. Retroreflective sheeting is commonly employed in a variety ofarticles, for example, road signs, barricades, license plates, pavementmarkers and marking tape, as well as retroreflective tapes for vehiclesand clothing.

[0004] Two known types of retroreflective sheeting are microsphere-basedsheeting and cube corner sheeting. Microsphere-based sheeting, sometimesreferred to as “beaded” sheeting, employs a multitude of microspherestypically at least partially embedded in a binder layer and havingassociated specular or diffuse reflecting materials (e.g., pigmentparticles, metal flakes or vapor coats, etc.) to retroreflect incidentlight. Cube corner retroreflective sheeting typically comprises a thintransparent layer having a substantially planar front surface and a rearstructured surface comprising a plurality of geometric structures, someor all of which include three reflective faces configured as a cubecorner element.

[0005] Cube corner retroreflective sheeting is commonly produced byfirst manufacturing a master mold that has a structured surface, suchstructured surface corresponding either to the desired cube cornerelement geometry in the finished sheeting or to a negative (inverted)copy thereof, depending upon whether the finished sheeting is to havecube corner pyramids or cube corner cavities (or both). Known methodsfor manufacturing the master mold include pin-bundling techniques,direct machining techniques, and techniques that employ laminae.

[0006] In pin bundling techniques, a plurality of pins, each having ageometric shape such as a cube corner element on one end, are assembledtogether to form a master mold. U.S. Pat. Nos. 1,591,572 (Stimson) and3,926,402 (Heenan) provide illustrative examples.

[0007] In direct machining techniques, a series of grooves are formed inthe surface of a planar substrate (e.g. metal plate) to form a mastermold comprising truncated cube corner elements. In one well knowntechnique, three sets of parallel grooves intersect each other at 60degree included angles to form an array of cube corner elements, eachhaving an equilateral base triangle (see U.S. Pat. No. 3,712,706(Stamm)). In another technique, two sets of grooves intersect each otherat an angle greater than 60 degrees and a third set of groovesintersects each of the other two sets at an angle less than 60 degreesto form an array of canted cube corner element matched pairs (see U.S.Pat. No. 4,588,258 (Hoopman)). In direct machining, a large number ofindividual faces are typically formed along the same groove formed bycontinuous motion of a cutting tool. Thus, such individual facesmaintain their alignment throughout the mold fabrication procedure. Forthis reason, direct machining techniques offer the ability to accuratelymachine very small cube corner elements. A drawback to direct machiningtechniques, however, has been reduced design flexibility in the types ofcube corner geometries that can be produced, which in turn affects thetotal light return.

[0008] In techniques that employ laminae, a plurality of thin sheets(i.e. plates) referred to as laminae having geometric shapes formed onone longitudinal edge are assembled to form a master mold. Laminatechniques are generally less labor intensive than pin bundlingtechniques because fewer parts are separately machined. For example, onelamina typically comprises about 400-1000 individual cube cornerelements in comparison to each pin comprising a single cube cornerelement. Illustrative examples of lamina techniques can be found in EP 0844 056 A1 (Mimura); U.S. Pat. No. 6,015,214 (Heenan); U.S. Pat. No.5,981,032 (Smith); U.S. Pat. No. 6,159,407 (Krinke) and U.S. Pat. No.6,257,860 (Luttrell).

[0009] The base edges of adjacent cube corner elements of truncated cubecorner arrays are typically coplanar. Other cube corner elementstructures, described as “full cubes” or “preferred geometry (PG) cubecorner elements” typically comprise at least two non-dihedral edges thatare not coplanar. Such structures typically exhibit a higher total lightreturn in comparison to truncated cube corner elements. Certain PG cubecorner elements may be fabricated via direct machining of a sequence ofsubstrates, as described in WO 00/60385. However, it is difficult tomaintain geometric accuracy with this multi-step fabrication process.Design constraints may also be evident in the resulting PG cube cornerelements and/or arrangement of elements. By contrast, pin bundling andtechniques that employ laminae allow for the formation of a variety ofshapes and arrangements of PG cube corner elements. Unlike pin bundling,however, techniques that employ laminae also advantageously provide theability to form relatively smaller PG cube corner elements.

[0010] After manufacturing a master mold the master mold is typicallyreplicated using any suitable technique such as conventional nickelelectroforming to produce a tool of a desired size for formingmicrostructured sheeting. Multigenerational positive and negative copytools are thus formed, such tools having substantially the same degreeof precise cube formation as the master. Electroforming techniques suchas described in U.S. Pat. Nos. 4,478,769 and 5,156,863 (Pricone) as wellas U.S. Pat. No. 6,159,407 (Krinke) are known. A plurality ofreplications are often joined together for example by welding such asdescribed in U.S. Pat. No. 6,322,652 (Paulson). The resulting toolingmay then be employed for forming cube corner retroreflective sheeting byprocesses such as embossing, extruding, or cast-and-curing, as known inthe art.

[0011] For example, U.S. Pat. Nos. 3,684,348 and 3,811,983 describeretroreflective material and a method of making a composite materialwherein a fluid molding material is deposited on a molding surfacehaving cube corner recesses and a preformed body member applied thereto.The molding material is then hardened and bonded to the body member. Themolding material may be a molten resin and the solidification thereofaccomplished at least in part by cooling, the inherent nature of themolten resin producing bonding to the body member thereof.Alternatively, the molding material may be fluid resin havingcross-linkable groups and the solidification thereof may be accomplishedat least in part by cross-linking of the resin. The molding material mayalso be a partially polymerized resin formulation and wherein thesolidification thereof is accomplished at least in part bypolymerization of the resin formulation.

[0012] Various retroreflective sheeting comprising truncated cube cornerarrays have been commercially successful such as retroreflectivesheeting commercially available from 3M Company (“3M”), St. Paul, Minn.under the trade designation “3M Scotchlite Brand Reflective Sheeting3990 VIP”. Although described in the patent literature, retroreflectivesheeting comprising an array of full cubes or PG cube corner elementshas not been manufactured commercially or sold. In order to accommodatethe commercial success of retroreflective sheeting comprising an arrayof full cubes or PG cube corner elements, industry would find advantagein improved methods of making retroreflective sheeting comprising sucharrays.

SUMMARY OF THE INVENTION

[0013] A characteristic of certain tools for making retroreflectivesheeting comprising an array of full cube or PG cube corner cavities isthe presence of channels. The present inventor has found that thepresence of such channel as well as the orientation of such channelsrelative to the direction of relative motion of the tool in comparisonto the resin delivery system (e.g. the advancing tool) duringmanufacture of sheeting has a significant effect on the quality of thereplication as well as the rate of manufacturing the sheeting. Thepresent invention relates to a method of making retroreflective sheetingproviding a tool comprising a cube corner microstructured surface withat least one channel, advancing the tool in a direction such that thechannel is substantially parallel to the direction of the advancingtool, casting a moldable resin onto said tool surface, solidifying theresin forming a retroreflective sheet having a surface comprising cubecorner elements; and removing the sheet from the tool.

[0014] In another embodiment, the invention discloses retroreflectivesheeting comprising a pair of longitudinal peripheral edges and at leastone row of PG cube corner microstructures and at least one channelextending substantially parallel to the row; wherein the channel issubstantially parallel to the longitudinal peripheral edges of thesheeting. The longitudinal peripheral edges span the sheeting in itsmaximum dimension as manufactured. The sheeting is preferably providedin a roll-good.

[0015] In each of these embodiments the cube corner microstructures arepreferably PG cube corner microstructures. The cube cornermicrostructures may be cavities or elements. The channel may be aprimary groove channel comprising a first planar face and second planarface intersecting at a common vertex. Alternatively or in additionthereto, the channel may be a structured channel comprising a first facecomprising cube corner faces and second face comprising opposing cubecorner faces. Alternatively, the structured channel may comprise theintersection of opposing non-dihedral edges of opposing cube cornermicrostructures. Alternatively, or in addition thereto, the channel maybe a cube cavity channel comprising a first planar face and a secondstructured face. The first planar face is preferably a replica of aprimary groove face. A replica of a cube cavity channel provides cubecorner elements. The resin may be a thermoplastic resin provided moltenor provided in the form of sheet. The (e.g. thermosetting, radiationcurable) resin may optionally be provided on a carrier web.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In the several figures of the attached drawing, like parts bearlike reference numerals, and:

[0017]FIG. 1 is a perspective view of an exemplary single lamina priorto formation of cube corner microstructures.

[0018]FIG. 2 is a perspective view of a master tool comprising fourlaminae comprising cube corner elements microstructures.

[0019]FIG. 3 is a perspective view of a tool that is a replica of themaster tool of FIG. 2 comprising cube corner cavity microstructures.

[0020]FIG. 4a is a side view of an exemplary method of extruding moltenpolymeric resin onto a tool with a slot die according to the presentinvention.

[0021]FIG. 4b is an enlarged view of the tool.

[0022]FIG. 4c is an enlarged view of the resin on the tool.

[0023]FIG. 5 is a side view of an exemplary slot die apparatus for usein the method of the invention.

[0024]FIG. 6 depicts a detailed side view of an exemplary slot dieapparatus for use in the present invention.

[0025]FIGS. 7a-7 d depict photographs of retroreflective sheetingprepared with an exemplary method and exemplary slot die apparatus ofthe invention.

[0026]FIG. 8 depicts retroreflective sheeting prepared from toolinghaving down-web channels in comparison to crossweb channels.

[0027]FIG. 9a depicts a photograph of retroreflective sheetingmanufactured wherein the channels were oriented crossweb (i.e.perpendicular) relative to the direction of motion of the tool.

[0028]FIG. 9b depicts a photograph of retroreflective sheetingmanufactured wherein the channels were orientated downweb (i.e.perpendicular) relative to the direction of motion of the tool.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] The method and apparatus of the invention relate to makingmicrostructured sheeting articles such as retroreflective sheeting.

[0030] As used herein, “sheeting” refers to a thin piece of polymeric(e.g. synthetic) material. The sheeting may be of any width and length,such dimension only being limited by the equipment (e.g. width of thetool, width of the slot die orifice, etc.) from which the sheeting wasmade. The thickness of retroreflective sheeting typically ranges fromabout 0.004 inches (0.1016 mm) to about 0.10 inches (2.54 mm).Preferably the thickness of retroreflective sheeting is less than about0.020 inches (0.508 mm) and more preferably less than about 0.014 inches(0.3556 mm). In the case of retroreflective sheeting, the width istypically at least 30 inches (122 cm) and preferably at least 48 inches(76 cm). The sheeting is typically continuous in its length for up toabout 50 yards (45.5 m) to 100 yards (91 m) such that the sheeting isprovided in a conveniently handled roll-good. Additional layers such asseal films or overlays may also be utilized. Alternatively, however, thesheeting may be manufactured as individual sheets rather than as aroll-good. In such embodiments, the sheets preferably correspond indimensions to the finished article. For example, the retroreflectivesheeting, may have the dimensions of a standard U.S. sign (e.g. 30inches by 30 inches (76 cm by 76 cm) and thus the microstructured toolemployed to prepare the sheeting may have about the same dimensions.Smaller articles such as license plates or reflective buttons may employsheeting having a correspondingly smaller dimension.

[0031] Regardless of whether the retroreflective sheeting is provided asa roll-good or as a sheet, the sheeting comprises a pair of longitudinalperipheral edges such as depicted by 2 a and 2 b of FIG. 8. Suchlongitudinal peripheral edges typically span the sheeting in a maximumdirection. Further the longitudinal peripheral edges are parallel withthe direction of motion of the advancing tool and/or advancing moldableresin from the process in which the sheeting was made. Preferably, rowsof PG cube corner elements are aligned parallel to these longitudinalperipheral edges.

[0032] As used herein, “microstructured” refers to at least one majorsurface of the sheeting comprising structures having a lateral dimension(e.g. distance between groove vertices of the cube corner structures) ofless than 0.25 inches (6.35 mm), preferably less than 0.125 inches(3.175 mm) and more preferably less than 0.04 inches (1 mm). The lateraldimension, particularly of cube corner elements, is preferably less than0.020 inches (0.508 mm) and more preferably less than 0.007 inches(0.1778 mm). The microstructures have an average height ranging fromabout 0.001 inches (0.0254 mm) to 0.010 inches (0.254 mm), with a heightof less than 0.004 inches (0.1016 mm) being most typical. Further, thesmallest lateral dimension of a cube corner microstructure us typicallyat least 0.0005 inches (0.0127 mm). Cube corner microstructures maycomprise either cube corner cavities or, preferably, cube cornerelements having peaks.

[0033] As used herein, “casting” refers to forming a moldable resin intoa sheet having a microstructured surface by contacting the moldableresin with a microstructured mold surface. The moldable resin ispreferably sufficiently fluid such that it may be extruded, pumped orpoured onto a molding tool having the microstructured surface. Theviscosity of the resin may vary widely. Polymerizable resins are oftenlow to moderate viscosity liquids, whereas thermoplastic resins may berelatively viscous at the casting temperature. Alternatively themoldable resin may be provided in the form of a sheet that is contactedwith an advancing embossing tool or by rolling bank processes thatinvolve the contacting a coated carrier web with a tool.

[0034] The tool used herein is typically obtained by first manufacturinga master mold that has a microstructured surface. Method ofmanufacturing master molds are known. Master molds employed for makingretroreflective sheeting are typically prepared from pin-bundlingtechniques, direct machining techniques, and techniques that employlaminae, as described in the art. The master mold for use in theinvention is preferably derived from a laminae technique.

[0035] With reference to FIG. 1, lamina 10 includes a first majorsurface 12 and an opposing second major surface (not shown). Lamina 10further includes working surface 16 and an opposing bottom surfaceextending between first major surface 12 and second major surface.Lamina 10 further includes a first end surface 20 and an opposing secondend surface 22.

[0036] Lamina 10 can be characterized in three-dimensional space withthe same superimposed Cartesian coordinate system. A first referenceplane 24 is centered between major surfaces 12. First reference plane24, referred to as the x-z plane, has the y-axis as its normal vector. Asecond reference plane 26, referred to as the x-y plane, extendssubstantially coplanar with working surface 16 of lamina 10 and has thez-axis as its normal vector. A third reference plane 28, referred to asthe y-z plane, is centered between first end surface 20 and second endsurface 22 and has the x-axis as its normal vector.

[0037] In the method of machining lamina comprising cube cornermicrostructures a first groove set, an optional second groove set, andpreferably a third primary groove are formed with a groove-formingmachine. As used herein, the term “groove set” refers to grooves formedin working surface 16 of the lamina 10 that range from being nominallyparallel to non-parallel to within 1° to the adjacent grooves in thegroove set. Alternatively or in addition thereto the grooves of thegroove set may range from being nominally parallel to non-parallel towithin 1° to particular reference planes as will subsequently bedescribed. Accordingly, each characteristic with regard to an individualgroove and or the groove of a groove set (e.g. perpendicular, angle.etc.) will be understood to have this same degree of potentialdeviation. Nominally parallel grooves are grooves wherein no purposefulvariation has been introduced within the degree of precision of thegroove-forming machine.

[0038] In general, the first groove set comprises a plurality of grooveshaving respective groove vertices that intersect the first major surface12 and working surface 16 of lamina. Although working surface 16 mayinclude a portion that remains unaltered (i.e. unstructured), it ispreferred that working surface 16 is substantially free of unstructuredsurface portions.

[0039] The second groove set, (i.e. when present) comprises a pluralityof grooves having respective groove vertices that intersect the firstmajor surface and the working surface 16 of lamina. The first and secondgroove sets intersect approximately along a first reference plane 24 toform a structured surface including a plurality of alternating peaks andV-shaped valleys. Although not depicted, this embodiment may appear thesame as the combination of lamina 200 and lamina 300 in FIG. 2.

[0040] Both the first and second groove sets may also be referred toherein as “side grooves”. As used herein, side groove refers to anindividual groove or a groove set wherein the groove(s) range from beingnominally parallel to non-parallel within 1°, per their respectivegroove direction vectors, to at least one adjacent groove and preferablyto all the grooves of the side groove set. The direction of a particulargroove is defined by a vector aligned with the groove vertex. The groovedirection vector may be defined by its components in the x, y, and zdirections, the x-axis being perpendicular to reference plane 28 andy-axis being perpendicular to reference plan 24. Alternatively or inaddition thereto, side groove refers to a groove that ranges from beingnominally parallel to reference plane 28 to nonparallel to referenceplane 28 to within 1°. Side grooves may optionally be perpendicular toreference plane 24 to this same degree of deviation. The side groovesmay comprise small purposeful variations for the purpose of improvingthe retroreflected divergence profile such as included angle errors,and/or skew, and/or inclination. The advantages of skew and/orinclination are described in U.S. patent application serial No.60/452464 filed Mar. 6, 2003; incorporated herein by reference. U.S.patent application serial No. 60/452464 was filed concurrently with U.S.patent application serial No. 60/452605, to which the presentapplication claims priority.

[0041] The lamina preferably comprises a primary groove face thatextends substantially the full length of the lamina. Formation of aprimary groove face results in a structured surface that includes aplurality of cube corner elements having three perpendicular orapproximately perpendicular optical faces on the lamina. Typically, theintersection of such primary groove face with either working surface 12or 14 is nominally parallel to reference plane 24 and 26. A singlelamina may have a single primary groove face, a pair of groove faces onopposing sides and/or a primary groove along the intersection of workingsurface 16 with reference plane 24 that concurrently provides a pair ofprimary groove faces. A pair of single laminae with opposingorientations and preferably multiple laminae (e.g. four laminaidentified as 100, 200, 300 and 400 in FIG. 2) with opposingorientations are typically assembled such that their respective primarygroove faces form a primary groove 52, for example as depicted withreference to FIG. 2.

[0042] To form a master tool of suitable size for formingretroreflective sheeting, a plurality of toolings (also referred to astiles) are formed by electroplating the surface of the master tool toform negative copies, subsequently electroplating the negative copies toform positive copies, electroplating the positive copies to form asecond generation negative copies, etc. The positive copy has the samecube corner element structure as the master tool, whereas the negativecopy is the cube cavity replica. Accordingly, a negative copy tool (e.g.FIG. 3) is employed to make a positive copy (i.e. cube corner element)sheeting whereas, a positive copy tool (e.g. FIG. 2) is employed to makea negative copy (i.e. cube corner cavity) sheeting. Tiling such toolingstogether can then assemble a master tool of the desired size. In thepresent invention the toolings are typically tiled in the sameorientation such that the channels are substantially continuous betweenjoined tooling portions.

[0043] Regardless of the manner is which the tool was derived andregardless of whether the tool comprises cube corner elementmicrostructures wherein the faces intersect at a peak or cube cornercavity microstructures wherein the replication thereof forms a cubecorner element, the tool employed in the method of the inventioncomprises at least one and typically a plurality of channels. Thechannels are typically parallel to the rows of the cube cornermicrostructures (e.g. the cube corner microstructures formed onindividual laminae).

[0044] In one aspect, the channel is defined by a primary groove. Asdepicted in FIG. 2, a primary groove channel 52 is typically created bya pair of adjacent primary groove faces. Alternatively or in additionthereto, a primary groove channel may be present along the intersectionof working surface 16 with reference plane 24 per FIG. 1. The primarygroove faces typically intersect forming a line. This intersection mayalso be referred to as a vertex. Primary groove channels differ fromgrooves formed by direct machining of truncated cube corner arrays. Inone aspect, the vertex of any one direct-machined groove of truncatedcube corner arrays is typically intersected by grooves of other groovesets (e.g. formed at 60° to a first groove set) and groove faces are notcontinuous. In contrast, the bottom portion (e.g. vertex) of a primarygroove is typically not intersected by other grooves. Accordingly, thegroove faces of truncated cube corner arrays comprise a plurality ofopposing triangular faces, whereas primary groove faces comprisenon-triangular faces such as pentagons. In a truncated cube cornerarrays the tool comprises a plurality of elements or a plurality ofcavities, but not both in the same tool. In contrast, tools comprisingfull cube or PG cube corner arrays have a combination of cavities (e.g.cube corner cavities and other cavities) and protrusions (e.g. cubecorner elements and other structures). Typically, at least about 10% toas much as about 50% of the faces of the primary groove are continuousand above the groove vertex line.

[0045] In another aspect, in alternative or in addition to the presenceof a primary groove channel(s), the tool may comprise a channel 54 andpreferably a plurality of channels formed by the intersection of a pairof rows of microstructured elements such as microstructured elementshaving opposing orientations as depicted in FIG. 2. Channels of thistype will be referred to as “structured channels” herein. Suchstructured channels may be present between primary groove channels.Further, such structured channels are typically parallel to the primarygroove channel. For example, the tool may have primary groove channelsalternated with structured channels as depicted in FIG. 2. Structuredchannels do not intersect at a linear line. Rather the intersectiontypically comprises a plurality of line segments formed by peaks andvalleys, such as peaks and valleys alternating in a repeating pattern.The faces of the structured channels preferably comprise cube cornerfaces (e.g. from adjacent rows of cubes formed on adjacent laminae).

[0046] Since the retroreflective sheeting is a replica of the tool, thetool of FIG. 2 produces cube corner microstructures comprising cubecorner cavities. Accordingly, retroreflective sheeting comprising cubecorner element microstructures are derived from a tool comprising cubecorner cavities as depicted in FIG. 3. The channels 53 of this type oftool differ from that of the channels of FIG. 2. In this embodiment,each of the channels generally comprises a substantially continuousplanar face 51. This face may preferably be derived from the replicationof a primary groove face. Alternatively, this face may result from thereplication of a portion of a major surface (e.g. 12 of FIG. 1).Alternatively, this face may be discontinuous wherein the face isinterrupted by structures intersecting with such face. The second faceof the channel is typically a structured face. In contrast to thestructured face depicted in FIG. 2, the structures of the second face ofFIG. 3 as depicted on the tool are not cube corner elements structures.However, the channel formed between planar face 51 and the secondstructured face forms a cube corner cavity, meaning that the replicationof the cavity forms cube corner elements (e.g. in a row). Thus, thesechannels may be referred to a cube corner cavity channels.

[0047] Channels can be further characterized with respect to a ratio ofthe depth of a channel relative the overall height of themicrostructured surface, i.e. distance along the z-axis between thehighest and lowest points of the microstructures. Although this ratiocan vary, the ratio is preferably greater than about 0.4 (e.g. 0.5, 0.6,0.7, 0.8, 0.9, approaching 1) to obtain good replication at speeds of atleast 10 feet/min.

[0048] In general, there is typically at least one channel for each rowof elements. Each channels typically extends the entire length of alamina, entire length of a row of cubes, the entire dimension of anarray, or the entire length of the sheeting. However, at minimum, thechannels extend for a length of at least about 10× its width, preferablyat least 100× its width and more preferably at least 500× its width.Regardless of the type of channel, i.e. primary groove channel,structured channel, or cube corner cavity channel, the method of theinvention employs providing the tool in a delivery system (e.g.dispensing device such as a slot die apparatus) such that a majorportion of such channels and preferably substantially the totality ofthe channels are substantially parallel to the relative motion of thetool in comparison to the delivery system, i.e. direction of theadvancing tool and/or advancing dispensing apparatus. Substantiallyparallel as it relates to the direction or orientation of the channelsrelative to the direction of filling of the channel refers to the acuteangle formed by these two directions. Preferably, the orientation of thechannel(s) does not vary from 0° by more than 20°, and more preferablyby no more than 10° (e.g. 9°, 8°, 7°, 6°), and most preferably by nomore than 5° (e.g. 5°, 4°, 3°, 2°, 1°). The method of the invention isdescribed with reference to the use of a slot die apparatus as adispensing means for providing the moldable (e.g. fluid) resin. As usedherein, “slot die apparatus” refers to an apparatus comprising a cavitythat includes a resin distribution portion, the arrangement of which canbe of various designs (e.g. coat hanger, T-slot, etc.), wherein thecavity terminates in a slot orifice provided between a pair of die lips.The slot orifice is typically rectangular. Slot die apparatus aretypically equipped with various other components such as adjustingbolts, electrical heaters, thermocouples, etc. as are known in the art.The dimensions of the slot orifice may vary. For example the width mayvary from 0.010 inches to 0.1 inches, whereas the length may vary from 2inches to 60 inches (i.e. width of the coating line).

[0049] Other dispensing apparatus may alternatively be employed in placeof the exemplary slot die apparatus described herein. For example one ormore needle delivery system may be employed. Further the moldable resinmay alternatively be provided as a sheet that is contact with anembossing tool (i.e. at least one of which is advancing) or the moldableresin may be provided on a carrier web such as in the case of rollingbank processes. Depending on the dispensing apparatus, the orifice maybe in close proximity to the tool surface or somewhat removed.

[0050] For example, the tool may be employed as an embossing tool toform retroreflective articles, such as described in U.S. Pat. No.4,601,861 (Pricone). Alternatively, the retroreflective sheeting can bemanufactured as a layered product by casting the cube-corner elementsagainst a preformed film as taught in PCT application No. WO 95/11464and U.S. Pat. No. 3,684,348, or by laminating a preformed film topreformed cube-corner elements.

[0051] The method of making retroreflective sheeting via casting ahardenable fluid synthetic resin, in the absence of the inventiondescribed herein is generally known from for example U.S. Pat. Nos.3,811,983 (Rowland); 3,689,346 (Rowland); and U.S. Pat. No. 5,961,846(Benson Jr.).

[0052] With reference to FIGS. 4a-4 c, a representative manufacturingapparatus and process 1010 includes advancing a tool 1200 having amicrostructured surface 1100, by means for example of drive rolls 1400 aand/or 1400 b; casting a fluid synthetic resin onto the microstructuredsurface of the tool with a slot die apparatus 1000; allowing the resinto sufficiently harden (i.e. solidify) while in contact with the toolforming a sheet 1600; and removing the sheet from the tool. In the caseof continuous production, the leading edge of the sheeting is removedfrom the tool surface with for example stripper roll 1800. The directionof filling of the tool is the direction of relative motion 310 of thetool relative to the delivery system (e.g. slot die apparatus).Accordingly, in FIG. 4a-4 c, the direction of filling is normal to theorifice of the dispensing apparatus.

[0053] Although the slot die apparatus and advancing tool are depictedin a vertical arrangement, horizontal or other arrangements (i.e. anglesbetween horizontal and vertical) may also be employed. Regardless of theparticular arrangement, the slot die apparatus provides the fluid resinto the microstructured tool at the orifice of the slot die apparatus,preferably in a direction normal to the tool. In addition, themanufacturing process may include multiple slot die apparatusarrangements. For example, a first slot die apparatus may be provided topartially fill the cube cavities followed by a second slot die providedto fill the remainder of the cavity.

[0054] The die is mounted in a substantial mechanical framework that iscapable of being moved towards the advancing tool surface by suitablemeans such as jackscrews or hydraulic cylinders. Alternatively, the diemay be stationary and the advancing tool surface moved towards the die.When the die is about 0.020 inches from the tool, the fluid syntheticresin (e.g. molten thermoplastic polymeric material) contacts the toolforming a continuous layer of the resin on the microstructured toolsurface. The gap between the slot die apparatus and the tool surface istypically less than about two times that of the final sheetingthickness. Accordingly, the gap ranges from about 0.004 inches to 0.030inches when producing sheeting with a nominal thickness of 0.0025 inchesto 0.015 inches.

[0055] The resin is of a viscosity such that it flows, optionally withapplied vacuum, pressure, temperature, ultrasonic vibration, ormechanical means, into the cavities in the molding surface. It ispreferably applied in sufficient quantity that it substantially fillsthe cavities. In a preferred embodiment, the fluid resin is delivered ata rate such that the final land thickness of the sheeting (i.e. thethickness excluding that portion resulting from the replicatedmicrostructure, 1300 b in FIG. 4c) is between 0.001 and 0.100 inches andpreferably between 0.003 and 0.010 inches. With reference to FIG. 4c,the surface 1300a of the resin (e.g. solidified) opposing the toolsurface is generally smooth and planar. Alternatively, however, theresin may be delivered in a manner such that the cube cavities alone arefilled and thus the sheeting is substantially free of a land layer. Inthis embodiment the cube corner elements are typically bonded to a filmlayer prior to removal from the tool surface.

[0056] In the case of extrusion of molten thermoplastic resins, theresin is typically initially provided in a solid pellet form and pouredinto hopper 2100 that continuously feeds the resin into a melt extruder2000. Heat is typically supplied to the tool by passing over the driveroll 1400A that is heated for example with circulating hot oil or byelectric induction to maintain a tool surface temperature above thesoftening point of the polymer. Suitable cooling means such as sprayingwater onto the extruded resin or tool, contacting the unstructuredsurface of the tool with cooling rolls, or direct impingement air jetsprovided by high-pressure blowers are provided after extrusion tosufficiently harden the resin such that it may be removed from the tool.

[0057] In the case of polymerizable resins, the resin may be poured orpumped directly into a dispenser that feeds slot die apparatus 1000. Forembodiments wherein the polymer resin is a reactive resin, the method ofmanufacturing the sheeting further comprises curing the resin in one ormore steps. For example the resin may be cured upon exposure to asuitable radiant energy source such as actinic radiation, ultravioletlight, visible light, etc. depending upon the nature of thepolymerizable resin to sufficiently harden the resin prior to removalfrom the tool. Combinations of cooling and curing may also be employed.

[0058] With reference to FIGS. 5-6, an exemplary slot die apparatus 1020for use in the invention comprises two portions, a first die portion 110and a second die portion 115. The first and second die portions arejoined together at the die parting line 180 creating a slot cavity (notshown) having a rectangular slot orifice 181. Adjacent to the slotorifice 181 and downstream of it relative to the direction of rotation310 of roll 1400 a, is a first die lip 120, also referred to herein asthe downstream lip. Adjacent to the slot orifice 181, and upstream of itrelative to the direction of rotation 310 of roll 1400 a, is a seconddie lip 170, also referred to herein as the upstream lip. These lips arebrought into close proximity to the continuously advancing moving tool1200 having a microstructured surface. The drive roll 1400 a is built toresist high die loading forces while maintaining overall roll surfacedeflection of less than 0.001 inches over the working face of the roll.

[0059] In the method of the present invention, the tool and/or moldableresin is advanced and thus provided such that the channels (i.e. primarygroove channels and/or structured channels and/or cube corner cavitychannel) of the tool are substantially parallel to the direction of theadvancing tool. In doing so the channels of the tool are preferablysubstantially normal to the slot orifice of the slot die apparatus. Thepresent inventor has discovered that providing the tool to thedispensing apparatus in a manner wherein the channels are perpendicularto the direction of the advancing tool results in poor tool filling.Poor tool filling is evident by the sheeting having an irregularappearance in plan view such as crossweb striations as depicted byportion A of FIG. 8 rather than being substantially free of suchirregularities as depicted by portion B of FIG. 8. Upon viewing thesheeting with a microscope, it is evident that portion A has substantialunfilled inclusions as depicted in FIG. 9b, whereas the percentage ofunfilled inclusions in FIG. 9a are less than 1%. Portion A of FIG. 8would also exhibit poor (if any) retroreflected brightness due to theseverity of the cube corner element defects.

[0060] Methods of machining laminae and forming a master tooling fromlaminae are known, such as described in U.S. Pat. No. 6,257,860 (Lutrellet al.). For embodiments wherein the side grooves are substantially freeof skew and/or inclination, side grooves may be formed in a plurality ofstacked laminae, such as described in U.S. Pat. No. 6,257,860 (Lutrellet al.) and U.S. Pat. No. 6,159,407 (Krinke et al.). A preferred methodfor forming grooves on the edge of individual lamina (e.g. lamina havingside grooves comprising skew and/or inclination), assembling thelaminae, and replicating the microstructured surface of the assembledlaminae is described in U.S. patent application Ser. No. 10/383,039filed Mar. 6, 2003; incorporated herein by reference. U.S. patentapplication Ser. No. 10/383,039 was concurrently filed with U.S. patentapplication serial No. 60/452605, to which the present applicationclaims priority. Such methods describe machining cube cornermicrostructures on an exposed edge surface portion of the lamina, (i.e.working surface 16 with reference to FIG. 1) by forming a plurality ofV-shaped grooves with a groove-forming machine.

[0061] A lamina is a thin plate having length and height at least about10 times its thickness (preferably at least 100, 200, 300, 400, 500times its thickness). The lamina(e) are not limited to any particulardimensions. One of ordinary skill in the art appreciates the optimaldimensions of the lamina are related to the flexural stiffness of thelamina, buckling stiffness, and ease of handling. Furthermore, optimaldimensions may also be constrained by the optical requirements of thefinal design (e.g. cube corner structures). The lamina typically has athickness of less than 0.25 inches (6.35 mm) and preferably less than0.125 inches (3.175 mm). The thickness of the lamina is preferably lessthan about 0.020 inches (0.508 mm) and more preferably less than about0.010 inches (0.254 mm). Typically, the thickness of a lamina is atleast about 0.001 inches (0.0254 mm) and more preferably at least about0.003 inches (0.0762 mm). Such laminae range in length from about 1 inch(25.4 mm) to about 20 inches (5.08 cm) and are typically less than 6inches (15.24 cm). The height of a lamina typically ranges from about0.5 inches (12.7 mm) to about 3 inches (7.62 cm) and is more typicallyless than about 2 inches (5.08 cm).

[0062] In general, the lamina may be comprised of any substrate suitablefor forming directly machined grooves on the edge. Suitable substratesmachine cleanly without burr formation, exhibit low ductility and lowgraininess and maintain dimensional accuracy after groove formation. Avariety of machinable plastics or metals may be utilized. Suitableplastics comprise thermoplastic or thermoset materials such as acrylicsor other materials. Machinable metals include aluminum, brass, copperelectroless nickel, and alloys thereof. Preferred metals includenon-ferrous metals. Suitable lamina material may be formed into sheetsby for example rolling casting chemical deposition, electro-depositionor forging. Preferred machining materials are typically chosen tominimize wear of the cutting tool during formation of the grooves. Othermaterials may also be suitable for lamina comprising other types ofmicrostructures.

[0063] The V-shaped grooves are preferably formed with a diamond-toolingmachine that is capable of forming each groove with fine precision.Moore Special Tool Company, Bridgeport, Conn.; Precitech, Keene, N.H.;and Aerotech Inc., Pittsburg, Pa., manufacture suitable machines forsuch purpose. Such machines typically include a laserinterferometer-positioning device. A suitable precision rotary table iscommercially available from AA Gage (Sterling Heights, Mich.); whereas asuitable micro-interferometer is commercially available from ZygoCorporation (Middlefield, Conn.) and Wyko (Tucson, Ariz.) a division ofVeeco. The precision (i.e. point to point positioning) of themicrostructure (e.g. groove vertices spacing and groove depth) ispreferably at least as precise as ±500 nm, more preferably at least asprecise as ±250 nm and most preferably at least as precise as ±100 nm.The precision of the groove angle is at least as precise as ±2 arcminutes (±0.033 degrees), more preferably at least as precise as ±1 arcminute (±0.017 degrees), even more preferably at least at precise as ±½arc minute (±0.0083 degrees), and most preferably at least as precise as±¼ arc minute (±0.0042 degrees) over the length of the cut (e.g. thethickness of the lamina). Further, the resolution (i.e. ability ofgroove forming machine to detect current axis position) is typically atleast about 10% of the precision. Hence, for a precision of ±100 nm, theresolution is at least ±10 nm. Over short distances (i.e. 10 adjacentparallel grooves), the precision is approximately equal to theresolution. In order to consistently form a plurality of grooves of suchfine accuracy over duration of time, the temperature of the process ismaintained within ±0.1° C. and preferably within ±0.01° C.

[0064] The diamond tools suitable for use are of high quality such asdiamond tools that can be purchased from K&Y Diamond (Mooers, N.Y.) orChardon Tool (Chardon, Ohio). In particular, suitable diamond tools arescratch free within 0.010 inches (0.254 mm) of the tip, as can beevaluated with a 2000× white light microscope. Typically, the tip of thediamond has a planar portion ranging in size from about 0.00003 inches(0.000762 mm) to about 0.00005 inches (0.001270 mm). Further, thesurface finish of suitable diamond tools preferably have a roughnessaverage of less than about 3 nm and a peak to valley roughness of lessthan about 10 nm. The surface finish can be evaluated by forming a testcut in a machinable substrate and evaluating the test cut with amicro-interferometer, such as can be purchased from Wyko (Tucson,Ariz.), a division of Veeco.

[0065] The method may employ a variety of dispensing apparatus forcasting the moldable (e.g. fluid) resin onto the microstructured toolsurface. Suitable dispensing apparatus include the slot die apparatusdescribed in U.S. Pat. No. 5,067,432 (Lippert) as well as slot dieapparatus commercially available from Extrusion Dies, Inc., ChippewaFalls, Wis. under the trade designations “Ultracoat” and “Ultraflex”. Insome embodiments the method of the invention further employs the methodsand apparatus described in attorney docket no. 58302US002, titled“Method of Making Retroreflective Sheeting and Slot Die Apparatus”,filed on the same day as the present application, incorporated herein byreference.

[0066] With reference to FIG. 6 an exemplary die comprises a downstreamlip having a total length of about 1.0 inch (e.g. 0.88 inches) havingtwo surface portions. The first surface portion extends from the leadingedge 130 to the trailing edge of the first surface portion 125 (i.e.also the leading edge of the second surface portion and the line ofadjacency for the two surface portions) having a length 121 a of 0.41″and an angle 128 of 89.2° degrees to vertical as measuredcounterclockwise from the lip surface to an extrapolation of die partingline 180, as depicted in FIG. 6. The second surface portion extends fromthe trailing edge of the first surface portion 125 having a length 126 aof 0.4741 and an angle 122 of 86.8° degrees to vertical as measuredcounterclockwise from the lip surface to an extrapolation of die partingline 180, as depicted in FIG. 3. Further, the downstream lip comprisessufficient structural strength to resist flexing or deflection due tothe high pressure developed between the lip surface portions and theadvancing tool.

[0067] The method of the invention is suitable for use with anymicrostructure design, e.g. cube corner element design comprisingprimary grooves and/or microstructured channels and/or cube cornercavity channels as previously described. Since the tool is provided suchthat a major portion of the channels are substantially parallel to thedirection of the advancing tool, in the case of roll-goods the channelsis also substantially parallel to the longitudinal edges, i.e. maximumdimension, of the sheet.

[0068] The retroreflective sheeting preferably comprises an array ofcube corner microstructures wherein at least a portion and preferablysubstantially all the cube corner elements of the lamina(e) andretroreflective sheeting are full cubes that are not truncated. In oneaspect, the base of full cube elements are not triangular. In anotheraspect, the non-dihedral edges of full cube elements arecharacteristically not all in the same plane (i.e. not coplanar). Suchcube corner elements are preferably “preferred geometry (PG) cube cornerelements”. Full-cube and PG cube corner elements typically exhibit ahigher total light return in comparison to truncated cube cornerelements.

[0069] A PG cube corner element may be defined in the context of astructured surface of cube corner elements that extends along areference plane. For the purposes of this application, a PG cube cornerelement means a cube corner element that has at least one non-dihedraledge that: (1) is nonparallel to the reference plane; and (2) issubstantially parallel to an adjacent non-dihedral edge of a neighboringcube corner element. A cube corner element whose three reflective facescomprise rectangles (inclusive of squares), rectangles, quadrilaterals,trapezoids pentagons, or hexagons are example of PG cube cornerelements. “Reference plane” with respect to the definition of a PG cubecorner element refers to a plane or other surface that approximates aplane in the vicinity of a group of adjacent cube corner elements orother geometric structures, the cube corner elements or geometricstructures being disposed along the plane. In the case of a singlelamina, the group of adjacent cube corner elements consists of a singlerow or pair of rows. In the case of assembled laminae, the group ofadjacent cube corner elements includes the cube corner elements of asingle lamina and the adjacent contacting laminae. In the case ofsheeting, the group of adjacent cube corner elements generally covers anarea that is discernible to the human eye (e.g. preferably at least 1mm²) and preferably the entire dimensions of the sheeting.

[0070] Suitable resin compositions for the retroreflective sheeting ofthis invention are preferably transparent materials that aredimensionally stable, durable, weatherable, and readily formable intothe desired configuration. Examples of suitable materials includeacrylics, which have an index of refraction of about 1.5, such asPlexiglas brand resin manufactured by Rohm and Haas Company;polycarbonates, which have an index of refraction of about 1.59;reactive materials such as thermoset acrylates and epoxy acrylates;polyethylene based ionomers, such as those marketed under the brand nameof SURLYN by E. I. Dupont de Nemours and Co., Inc.;(poly)ethylene-co-acrylic acid; polyesters; polyurethanes; and celluloseacetate butyrates. Polycarbonates are particularly suitable because oftheir toughness and relatively high refractive index, which generallycontributes to improved retroreflective performance over a wider rangeof entrance angles. Injection molding grade polycarbonate having a meltflow rate ranging from 17 g/10 min. to 24 g/ 10 min. (ASTM D1238 or ISO1133-1991; condition 300/1.2) is typically preferred. These materialsmay also include dyes, colorants, pigments, UV stabilizers, or otheradditives. Although transparent synthetic resins are employed in themanufacture of retroreflective sheeting, in the case of othermicrostructured articles, the synthetic resin may be opaque ortranslucent as well.

[0071] In the case of molten polymeric resins, the resin typicallysolidifies as a function of sufficient cooling. For example,polycarbonate sufficiently cools upon reaching a temperature of about240° F. or lower. Cooling can be achieved by any means including byspraying water onto the extruded resin or tool, contacting theunstructured surface of the resin or tool with cooling rolls, or mymeans of direct impingement air jets provided by high-pressure blowers.

[0072] Other illustrative examples of materials suitable for forming thearray of cube corner elements are reactive resin systems capable ofbeing cross-linked by a free radical polymerization mechanism byexposure to actinic radiation, for example, electron beam, ultravioletlight, or visible light. Additionally, these materials may bepolymerized by thermal means with the addition of a thermal initiatorsuch as benzoyl peroxide. Radiation-initiated cationically polymerizableresins also may be used. Reactive resins suitable for forming the arrayof cube corner elements may be blends of photoinitiator and at least onecompound bearing an acrylate group. Preferably the resin blend containsa monofunctional, a difunctional, or a polyfunctional compound to ensureformation of a cross-linked polymeric network upon irradiation.

[0073] Illustrative examples of resins that are capable of beingpolymerized by a free radical mechanism that can be used herein includeacrylic-based resins derived from epoxies, polyesters, polyethers, andurethanes, ethylenically unsaturated compounds, isocyanate derivativeshaving at least one pendant acrylate group, epoxy resins other thanacrylated epoxies, and mixtures and combinations thereof. The termacrylate is used here to encompass both acrylates and methacrylates.U.S. Pat. No.4,576,850 (Martens) discloses examples of crosslinkedresins that may be used in cube corner element arrays of the presentinvention.

[0074] The manufacture of the sheeting may include other optionalmanufacturing steps prior to or subsequent to solidification of thesheeting. For example, the retroreflective sheeting can be manufacturedas a layered product by casting the cube-corner elements against apreformed film as taught in PCT application No. WO 95/11464 and U.S.Pat. No. 3,684,348, or by laminating a preformed film to preformedcube-corner elements. In doing so the individual cube-corner elementsare interconnected by the preformed film. Further, the elements and filmare typically comprised of different materials.

[0075] Alternatively or in addition thereto, specular reflective coatingsuch as a metallic coating can be placed on the backside of thecube-corner elements. The metallic coating can be applied by knowntechniques such as vapor depositing or chemically depositing a metalsuch as aluminum, silver, or nickel. A primer layer may be applied tothe backside of the cube-corner elements to promote the adherence of themetallic coating.

[0076] In addition to or in lieu of a metallic coating, a seal film canbe applied to the backside of the cube-corner elements; see, forexample, U.S. Pat. Nos. 4,025,159 and 5,117,304. The seal film maintainsan air interface at the backside of the cubes that enables totalinternal reflection at the interface and inhibits the entry ofcontaminants such as soil and/or moisture. Further a separate overlayfilm may be utilized on the viewing surface of the sheeting for improved(e.g. outdoor) durability or to provide an image receptive surface.Indicative of such outdoor durability is maintaining sufficientbrightness specifications such as called out in ASTM D49560-1a afterextended durations of weathering (e.g. 1 year, 3 years). Further the CAPY whiteness is preferably greater than 30 before and after weathering.

[0077] The retroreflective sheeting is useful for a variety of uses suchas traffic signs, pavement markings, vehicle markings and personalsafety articles, in view of its high retroreflected brightness. Thecoefficient of retroreflection, R_(A), may be measured according to USFederal Test Method Standard 370 at −4° entrance, 0° orientation, 0.2°observation is typically at least 100 candelas per lux per square meter(CDL), preferably at least 300 CDL and more preferably at least 600 CDL.

[0078] Patents, patent applications, and publications disclosed hereinare hereby incorporated by reference (in their entirety) as ifindividually incorporated. It is to be understood that the abovedescription is intended to be illustrative, and not restrictive. Variousmodifications and alterations of this invention will become apparent tothose skilled in the art from the foregoing description withoutdeparting from the scope of this invention, and it should be understoodthat this invention is not to be unduly limited to the illustrativeembodiments set forth herein.

EXAMPLES 1-4

[0079] Retroreflective sheeting was prepared utilizing a slot dieapparatus that was substantially identical to a slot die apparatuscommercially available from Extrusion Dies, Inc., Chippewa Falls, Wis.under the trade designation “Ultracoat” with the exceptions that thedownstream die lip was changed to incorporate the following specificfeatures: (1) the horizontal length of the lip was changed from 0.47841to 0.88441 ; (2) a section of the lip near the polycarbonate exit slotwas thinned to provide a hinge in the horizontal section of the lip,thereby allowing the downstream portion of the lip to be adjusted in avertical plane; (3) the polycarbonate contacting surface of this lip wasmachined to provide two planar surfaces, the first surface being at anangle of 89.2 degrees to vertical as measured counterclockwise from thetool surface to an extrapolation of the parting line of the die, and thesecond surface being 87.6 degrees to vertical counterclockwise asmeasured counterclockwise from the tool surface to an extrapolation ofthe parting line of the die; (4) die bolts were configured to push onthe outboard or trailing section of the die lip, such that adjustmentsof the bolts resulted in a vertical displacement of the trailing lipwhile not substantially changing the polycarbonate slot between thefront and rear die lips; (5) the lip was built from P-20 tool steel suchthat with reference to FIG. 3 dimension 150 had a thickness of 0.33inches, dimension 151 had a thickness of 0.26 inches, dimension 152 hada thickness of 0.19 inches and angle 153 was 105° relative to referenceplane 2000.

[0080] In operation, the die was mounted such that the parting line slotorifice was positioned horizontally 0.05041 upstream of top dead centerrelative to a reference plane tangent to the roll at top dead center ina rigid framework having a die support beam and a jackscrew assembly formoving the die in a vertical plane so that it could be positioned at anydistance from a heated roller. The jackscrew assembly was equipped withactuating motors on both sides of the support beam that were mountedwith bolts to the die beam. The moving end of the jackscrew was threadedinto a load cell that was bolted to the main support structure in amanner that the net load developed by the molten resin interacting withthe die lips was transmitted through and sensed by the load cells. Theload cells were connected to suitable electronics that provided adigital display of these die forces. The die was attached to a singlescrew extruder.

[0081] Injection molding grade polycarbonate having a melt flow rateranging from 17 g/10 min. to 24 g/10 min. (ASTM D1238 or ISO 1133-1991;condition 300/1.2) was dried for 4 hours in a 250° F. drying hopper. Thedried polycarbonate pellets were flood fed to the extruder inlet. Theextruder barrel zone temperatures were set at 475° F. for Zone 1, 535°F. for Zone 2, 550° F. for Zone 3, 565° F. for Zone 4 and 570° F. forZone 5. The gate exit zone temperature at the end of the extruder wasset at 575° F. The polymer melt temperature and pressure were measuredat the extruder gate using a melt thermocouple and a pressure proberespectively and are provided in TABLE 1. An adapter of 1.25 inchinternal diameter (I.D.) connected the extruder gate and the die. Thetemperature on the adapter was set at 560° F. The die body was dividedinto 16 temperature zones, each temperature zone having approximatelythe same area. The downstream lip included sequential zones 1-8, whereasthe upstream lip included sequential zones 9-16 with zones 1 and 16being adjacent to one another yet on opposite sides of the slot orifice.The zone temperatures were set at 575° F. for Zones 1, 2, 7, 8, 9 and10; 560° F. for Zones 3, 6, 11 and 14; 545° F. for Zones 4, 5,12 and 13;and 570° F. for Zones 15 and 16.

[0082] The die was initially positioned so that the downstream lip wasapproximately 10 mils from the microprismatic surface of a tool (as willsubsequently be described in greater detail) that consisted of theinverse of the desired microprismatic design of the retroreflectivesheeting. The vertical position of the trailing die lip 126 was adjustedby turning all the die bolts ⅜ of one turn resulting in the trailingedge 140 of the second surface portion being 0.00441 closer to the tool.The microprismatic surface was on a continuous metal belt that was setat the line speed provided in Table 1. The microprismatic surface of thetool was presented to the die by wrapping the belt around a continuouslydriven heated roller built to resist high die loading forces whilemaintaining overall roller surface deflection less than 1 mil over theworking face of the roller having a diameter of 30 inches. Heat wassupplied to the roller by a hot oil system with a set point of 495° F.The tool surface temperature was measure with a contact pyrometer onboth the left and right flat non-structured margins of the tooling asprovided in TABLE 1.

[0083] The tool was about 20 feet in the downweb direction by about 3feet in the crossweb direction. The tool included electroformedreplications that were derived from a master mold consisting of anassembly of laminae having dimensions of about 2 inches by about 4inches (i.e. length of microprismatic surface of each lamina). Themethod of machining the lamina as well as the method of assembling andreplicating the assembled laminae is described in previously cited U.S.patent application Ser. No. 10/383,039 filed Mar. 6, 2003. The opticaldesign formed in the lamina(e) is described in previously cited U.S.patent application serial No. 60/452464. A primary groove face extendingsubstantially the full lamina length was formed on each lamina. Theprimary groove face was oriented at roughly 35.49° to the normal vectordefined by the plane of the tool surface. Alternating pairs of sidegrooves with included angles of substantially 75.226° and 104.774° wereformed in each lamina with a spacing of 0.005625 inches to produce theremaining faces of the cube corner cavities. The symmetry axes of thecubes corner cavities were canted about 6.03° in a plane substantiallyparallel to reference plane 24. The side grooves were formedsubstantially orthogonal to the primary groove face. The termsubstantially with regard to the side groove (i.e. included angles andorthogonal) refers to the side groove comprising a combination of ½angle errors, skew and inclination, each of which are less than 1° forthe purpose of introducing multiple non-orthogonality to improve theretroreflected divergence profile. Further details concerning groovescomprising skew and/or inclination is found in previously cited attorneydocket no. FN58179US002. The combination of ½ angle errors, skew andinclination is not believed to affect the replication fidelity.

[0084] Since the tool surface is a negative replica of the assembledlaminae, the tool comprises a plurality of substantially parallel cubecorner cavity channels. The tool was provided to the slot die apparatussuch the channel was normal to the slot orifice.

[0085] Photographs of retroreflective sheeting replicated from this toolare depicted in FIG. 7. The horizontal dimensions of the cube cavitiesin FIG. 7 is 0.0075 mils, corresponding to the thickness of theindividual lamina. The trapezoidal cube corner cavities of individuallamina correspond to the vertical rows in FIG. 7.

[0086] Molten polycarbonate exited from the die orifice onto themicroprismatic surface of the tooling to form a continuous web ofmicroreplicated sheeting. The extruder output speed was adjusted toprovide a 12 mil nominal caliper of the sheeting. The die force wasmeasured by the load cells built into the supporting framework.

[0087] The belt and web continued from the curved surface of the rollinto a flat free span zone and were then cooled by blowing air throughimpingement nozzles until a temperature of less than 240° F. wasreached. The web was then removed from the belt and wound into a roll ofretroreflective sheeting.

[0088] The replication fidelity of each retroreflective sheeting samplewas evaluated by taking four 3″×5″ samples from each of four locationsacross the web. Care was taken to ensure that the sixteen samples fromeach comparative example was taken from the same location. Each samplewas laid under a microscope (Measurescope MM-11) at a 10× magnificationand a photograph (camera was a Javelin SmartCam) was taken of the samplewith the poorest replication. The photographs are depicted in FIG. 7a-7d. Poor replication would be evident by the presence of unfilledinclusions of the cube cavities that appear as black clusters at thecenter portion of each trapezoid, each trapezoid being the base edges ofthe cube corner element. The percentage of unfilled inclusions isapproximated by measuring the surface area of the unfilled inclusions inplan view. A “pass” rating refers to 1% or less unfilled inclusions,whereas a “fail” rating refers to greater than 1% unfilled inclusions.TABLE 1 Example No. 1 2 3 4 Line Speed (fpm) 10 14 18 20 Extruder Speed(rpm) 7 9 11 13 Extruder Gate Melt 561 563 561 561 Temperature (° F.)Slot Pressure (psi) 886 730 785 661 Extruder Gate Melt 1789 1767 21642210 Pressure (psi) Tool Temperature (° F.) 425 405 400 398 Die Force(pli) 607 647 637 636 Replication Fidelity Pass Pass Pass Pass <1% loss<1% loss <1% loss <1% loss

[0089] Table 1 shows that each of Examples 1-4 exhibited goodreplication fidelity.

EXAMPLE 5 & COMPARATIVE EXAMPLE A

[0090] The same general procedure as described for Examples 1-4 wasrepeated with a different slot die apparatus wherein the total length ofthe downstream lip was 0.501 inches and the downstream lip comprised asingle planar surface. The tool comprised cube corner cavity channelshaving cube cavities replicated from laminae forward canted by about9.74°. The width of the cube cavities was 7.5 mils corresponding to thethickness of a lamina and the side groove spacing was 5.0 mils. The cubepeak was centered between the side grooves and located 4.125 mils inplan view form the side of the lamina intersected by the primary groovesurface. All side grooves having an included angle of substantially 90°.The primary groove face was oriented at roughly 45° to the normal vectordefined by the plane of the tool surface.

[0091] In a first section of the tool, the tool was provided to the slotdie apparatus such that the cube cavity channels were orientatedcrossweb relative to the direction of the advancing tool. Although thereplication was good at speeds less than 5 feet per minute, the toolfilling was poor at speeds greater than 5 feet per minute.

[0092] In a second section of the tool, the tool was provided to theslot die apparatus such that the cube cavity channels were orientateddownweb (i.e. channel parallel to the direction of the advancing tool)relative to the direction of the advancing tool. The replication wasgood at a range of speeds up to about 28 feet per minute.

[0093]FIG. 8 depicts a photograph of retroreflective sheeting. Section Ais a portion of the sheeting that was replicated at a rate of 11feet/minute with the channels of the tool orientated crossweb, whereasSection B is a portion of the sheeting that was replicated using thesame conditions except for the same tool being provided to the slot dieorifice such that the channels of the tool were orientation downweb.

[0094]FIG. 9a depicts a photograph of the retroreflective sheeting ofSection A, whereas FIG. 9b depicts a photograph of Section B. As shownin these photographs Section B had substantial unfilled inclusions.

What is claimed is:
 1. A method of making retroreflective sheetingcomprising: providing a tool comprising a PG cube corner microstructuredsurface with at least one channel; advancing the tool in a directionsuch that the channel is substantially parallel to the direction of theadvancing tool; casting a moldable resin onto said tool surface;solidifying the resin forming a retroreflective sheet having a surfacecomprising cube corner elements; and removing the sheet from the tool.2. The method of claim 1 wherein the cube corner microstructures arecavities.
 3. The method of claim 1 wherein the cube cornermicrostructures are cube corner elements.
 4. The method of claim 1wherein the channel is a primary groove channel.
 5. The method of claim4 wherein the channel comprises a first planar face and second planarface intersecting at a common vertex.
 6. The method of claim 1 whereinthe channel is a structured channel.
 7. The method of claim 6 whereinthe channel comprises a first face comprising cube corner faces andsecond face comprising opposing cube corner faces.
 8. The method ofclaim 6 wherein the channel comprises the intersection of opposingnon-dihedral edges of opposing cube corner microstructures.
 9. Themethod of claim 1 wherein the channel is a cube cavity channel.
 10. Themethod of claim 9 wherein the channel comprises a first planar face anda second structured face.
 11. The method of claim 9 wherein a replica ofthe channel provides cube corner elements.
 12. The method of claim 10wherein the first planar face is a replica of a primary groove face. 13.The method of claim 1 wherein the resin is a molten thermoplastic resin.14. The method of claim 1 wherein the resin is a thermoplastic resinprovided in the form of a sheet.
 15. The method of claim 1 wherein theresin is provided on a carrier web.
 16. A method of makingretroreflective sheeting comprising: providing a tool comprising a cubecorner microstructured surface with channels selected from primarygroove channels, structured channels, cube cavity channels, andcombinations thereof; advancing the tool in a direction such that thechannel is substantially parallel to the direction of the advancingtool; casting a moldable resin onto said tool surface; solidifying theresin forming a retroreflective sheet having a surface comprising cubecorner elements; and removing the sheet from the tool. 17.Retroreflective sheeting comprising at least one longitudinal peripheraledge and at least one row of PG cube corner microstructures and at leastone channel extending substantially parallel to the row; wherein thechannel is substantially parallel to the longitudinal peripheral edgesof the sheeting.
 18. The sheeting of claim 17 wherein the cube cornermicrostructures are PG cube corner elements.
 19. The sheeting of claim17 wherein the cube corner microstructures are PG cube corner cavities.20. The sheeting of claim 17 wherein the sheeting comprises athermoplastic resin.
 21. The sheeting of claim 17 wherein thelongitudinal peripheral edge spans the sheeting in a maximum dimension.22. The sheeting of claim 17 wherein the sheeting comprises a pair orparallel longitudinal peripheral edges.
 23. The sheeting of claim 17wherein the sheeting is provided in a roll-good.
 24. Retroreflectivesheeting comprising a pair of longitudinal peripheral edges and rows ofcube corner microstructures and channels extending substantiallyparallel to the rows; wherein a major portion of the channels aresubstantially parallel to the longitudinal peripheral edges of thesheeting and the channels are selected from primary groove channels,structured channels, cube cavity channels, and combinations thereof.